In this lesson, we will become acquainted with analog and digital signals, the characteristics of each, and how they are used in data transmission. We will also discuss why modulation and demodulation processes are required for sending and receiving data over transmission media.

Kent is an adjunct faculty member for the College of Business at Embry-Riddle Aeronautical University and has a Master's degree in Technical Management.

In this lesson, we will become acquainted with analog and digital signals, the characteristics of each, and how they are used in data transmission. We will also discuss why modulation and demodulation processes are required for sending and receiving data over transmission media.

What Are Analog and Digital Signals?

Have you ever sat by the phone waiting for it to ring? Well, I guess we don't sit by the phone anymore since we carry it with us wherever we go. Regardless, in anticipation of great news, we have all probably looked at the phone and mumbled, ''Come on, ring!'' When the telephone finally rang, we answered it, connecting to the individual with the good news on the other end. As we speak into a microphone, sound waves from our voice are transmitted by the telephone in the form of radio waves (both sound waves and radio waves are analog signals), and when combined with digital signals, we begin moving data along the telephone line. We will come back to our good news in a few minutes, but first let's define analog and digital signals and then discuss what happens during the conversation.

Analog Signal

An analog signal is a continuous wave denoted by a sine wave (pictured below) and may vary in signal strength (amplitude) or frequency (time). The sine wave's amplitude value can be seen as the higher and lower points of the wave, while the frequency (time) value is measured in the sine wave's physical length from left to right.

There are many examples of analog signals around us. The sound from a human voice is analog, because sound waves are continuous, as is our own vision, because we see various shapes and colors in a continuous manner due to light waves. Even a typical kitchen clock having its hands moving continuously can be represented as an analog signal.

Analog signal represented as a sine wave

Digital Signal

A digital signal - a must for computer processing - is described as using binary (0s and 1s), and therefore, cannot take on any fractional values. As illustrated in the graphic below, digital signals retain a uniform structure, providing a constant and consistent signal. Because of the inherent reliability of the digital signal, technology using it is rapidly replacing a large percentage of analog applications and devices. For example, the wristwatch, showing the time of day, with its minute, hour, and sweeping second hands, is being replaced by the digital watch, which offers the time of day and other information using a numerical display. A typical digital signal is represented below. Note the equally dispersed 1s and 0s.

Digital signal with binary

What Is a Modem?

Within a communications environment, a modem is a combination of two networking devices: a modulator and a demodulator (modem for short). These devices perform modulation and demodulation algorithms simultaneously, to convert a signal from analog-to-digital and digital-to-analog, enabling data transmission to and from various computing resources.

What Is Modulation/Demodulation

Let's look at a typical data flow using the illustration below as a reference. A good place to start is at the telephone, with you making a call. Remember, as we speak into a microphone, sound waves from our voice are transmitted by the telephone in the form of radio waves (analog signals), which characteristically vary in signal strength or frequency. The analog signals travel from the telephone into the modem. It is the modem's job to demodulate, that is transform analog signals into digital signals for computer processing, and then eventually modulate the signal, reverting the digital signals back into analog signals.

In other words, two modems are required to perform the analog-to-digital (demodulation) and the digital-to-analog (modulation) transformations. When your call is returned, the digital signal is received from the telephone company's telecommunications network and passes through your telephone line into the modem where the signal is modulated (converted to analog). This enables you to hear and hopefully understand the good news being passed by the caller. It is not a stretch to say that without modulation and demodulation technology, we would not be able to send and receive information over a common transmission media, such as your telephone line.

Modulation and Demodulation scenario

Which Signal Is Better?

As digital technology continues to advance, analog technology will be used less because digital signals are more reliable and are not significantly influenced by outside interference (called noise), like analog signals. This is why digital is recommended over analog when consistent signal flow is required. Another advantage, and perhaps the most desirable in today's communications, is that digital signals can be transmitted at a higher rate than analog signals, which provides an enhancement in speed.

On the other hand, analog signals do have their own advantages. One such advantage is in operating costs. Traditionally, analog equipment and associated media tends to be less expensive in their initial purchase, maintenance, and replacement costs. Also, analog signals can carry an infinite amount of data due to having greater density. Additionally, considering the lack of complexity in comparison to digital signals, analog signals are easier to control and process when compared to digital signals.

Considering both the advantages and disadvantages of both signal types, digital signals are considered to have an advantage over analog signals.

Lesson Summary

Let's review. Analog signals are denoted by the sine wave and vary in amplitude and time. We also discussed the digital signal and its use of binary (1s and 0s), and that modulation and demodulation (modem) are required for sending and receiving data over a transmission media. Advantages of digital signals include reliability and higher speed, while advantages of analog signals include less operating costs, the ability to carry an infinite amount of data, and ease of control and processing.

Digital vs. Analog

Modems receive and transform digital and analog signals

Digital

Analog

*Uses binary (0s and 1s) to relay information*Retains a uniform structure *Provides a clear and consistent signal *Can be transmitted very quickly

*Denoted by a continuous sine wave *May vary in signal strength and frequency *Carries an infinite amount of data

Learning Outcomes

When you are done, you should be able to:

Describe and compare analog and digital signals

Explain the role of a modem in relaying analog and digital signals

Summarize the advantages and disadvantages of analog and digital signals

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